This invention relates to an erosion resistant, high current, draw-out fuseholder having a first, electrically conductive metal fuse contact having an outer chamfer, a second, extended, electrically conductive metal fuse contact having an outer chamfer and an inner chamfer, and an insulating tube between the fuse contacts, where the top of the tube has a sharp edge and discontinuity near the second fuse contact inner chamfer. This fuseholder can be used in pad mounted and submersible distribution transformers.
Replaceable, under oil expulsion fuses are generally used in high voltage systems to protect electric devices from fault currents, and are disclosed in U.S. Pat. No. 4,320,375 (Lien). There, the fuse holder includes a glass wound tube, impregnated with epoxy resin, covering an inner pressure tube of a nontracking, nonconducting material, such as polytetrafluoroethylene (Teflon). This composite, tubular, insulating structure is disposed between and fitted flush with two electrically conductive contacts having similar lengths and configurations, each fuse contact having an outer chamfered surface. A metallic fuse element which will melt at a particular load current or temperature, to interrupt the circuit, extends through the interior of the hollow tubular structure between the contacts. The fuse holder is shown mounted in an open housing which is totally immersed in insulating oil. If spring loaded housing contacts are used which touch each of the same length fuse contacts perpendicular to the fuse contact surface, as ,is the case in certain types of housing arrangements, upon withdrawal of the fuseholder, the upper spring loaded housing contact can disengage first and an electric arc can form at the upper fuse contact, causing pitting at that surface.
Similar type expulsion fuses, having a glass epoxy-Teflon pressure tube between threadedly mounted metal contacts of similar length and configuration, each having a diameter substantially greater than the pressure tube, are disclosed in U.S. Pat. No. 4,625,196 (Muench et al.). In this patent, primarily directed to the fuse assembly, both metal contacts have a beveled inner chamfer so that the pressure tube substantially "blends" into the contacts. In a modification of this design, U.S. Pat. No. 4,628,292 (Muench et al.) discloses a single layer, glass epoxy pressure tube between threadedly mounted metal contacts, each having a diameter substantially greater than the pressure tube. In this patent, also primarily directed to the fuse assembly, one metal contact has a beveled inner chamfer and the other metal contact, which appears elongated, has a sharp inner edge, and contains both an inner pressure chamber and vent holes through the contact surface to the pressure chamber.
Earlier art had disclosed the use of fuseholders having two fuse contacts of the same length and configuration having inner chamfers smoothly mating to a central insulating tube, as shown in British Patent No. 563,600 (Sowood et al.); U.S. Pat. No. 2,781,434 (Swain); U.S. Pat. No. 3,222,482 (Hitchcock) and U.S. Pat. No. 3,979,709 (Healey, Jr.). U.S. Pat. No. 3,911,385 (Blewitt et al.) discloses contacts of the same length and configuration, but with an outward fuse contact extension at the juncture with the central insulating tube. U.S. Pat. No. 1,853,093 (Steinmayer) appears to disclose fuse contacts having different lengths and configurations, but the contacts cover most of the insulating tube and the longer contact butts against a porcelain flanged insulating member.
All such structures would not appear to solve arcing between a spring loaded housing contact and the circumferential top mating surface of the fuseholder contacts upon withdrawal of the fuseholder, which arcing causes pitting and erosion of the contact surfaces and the insulating tube. It is the object of this invention to solve such problems.